Personal digital devices, distributed electricity for the developing as well as developed world, electric vehicles, telecom equipment, space vehicles, and military field apparatus, to name only a few applications, all require and will benefit from a high energy density battery that is directly and efficiently self-recharging in sunlight or even ambient artificial light.
Batteries coupled with chargers that employ photovoltaic (PV) cells, panels or arrays are ubiquitous, but they do not improve the charging efficiency because they do nothing to change the oxygen evolution potential of the anode that utilizes some of the power that could have gone into charging. And silicon-based PV technology is not compatible with the environment in alkaline batteries and so they cannot be truly integral.
Fuel cells are making inroads in remote power and backup power applications, but face an uphill climb to replace batteries because they further require a hydrogen source that, if renewable, comprises at least photovoltaic (PV) panels, electrolyzer, purifier, compressor, and storage cylinder. All these discrete components must be managed, and together comprise an expensive, bulky and inelegant system with reliability issues arising from the integration of so many components. Moreover, the stack-up efficiency losses of these components (50% electrolyzer energy loss, 50% fuel cell energy loss, and 10% compression energy loss, and 15% solar to electricity PV conversion) results in less than 4% of the incoming sunlight being converted to electricity for use when there is no sunlight (as compared to 15% direct solar-to-electricity PV conversion efficiency).
Fuel cell systems further require water replenishment. While theoretically the water from the fuel cell exhaust can be condensed, captured and used for replenishment, in practice this is not very effective or efficient and requires additional hardware and management thereof. One can imagine the difficulty in replenishing water in a fuel cell system at the top of a telephone pole or telecomm tower, and in winter for example.
Some fuel cell manufacturers have developed fuel cells that can be operated in reverse as electrolyzers to produce their own hydrogen supply, and in some cases reasonable hydrogen compression can be achieved on board electrochemically as well, thereby removing the discrete electrolyzer and compressor components from the system. However, such reversible fuel cells still suffer from “round trip” efficiency of less than 25%, so when powered by 15% efficient PV, solar-to-electricity-to-hydrogen and back to electricity is again less than 4% efficient. This is because it is difficult to optimize the same device for operation as both fuel cell and electrolyzer. Other issues include high cost, difficulty to reduce package size, lifetime, and inadequate on-board hydrogen storage density as compressed gas compared to a discrete compressor and storage cylinder.
The compressor and cylinder in the first system example above can in principle also be replaced by use of discrete dry metal hydride (MH) for hydrogen storage, but still some amount of compression and/or heating are required for the metal hydride to absorb hydrogen. And because heating of the MH is required to release the hydrogen there is a parasitic loss (which may not be a problem when there is an adequate source of waste heat available, but this is unlikely in remote applications).
As is well known in the art, metal hydrides can be employed for storing electricity through electrochemical storage of hydrogen in Ni-MH cells and batteries comprised of such cells. When charging, such cells are essentially alkaline electrolyzers, except that the hydrogen produced is a means to electrical storage and generation, rather than an end product as with electrolyzers. When an electrical potential is applied, hydrogen is both produced and absorbed by the metal hydride (MH) cathode along with some-amount of oxygen evolution at the anode. The latter is, for battery purposes, a parasitic energy loss. Absorption, as well as release during discharge, of hydrogen by the metal hydride is governed by the electrical voltage (potential), and polarity applied to the cell electrodes. This is a much simpler, reliable, and more efficient way to charge the MH with hydrogen than is the case for dry external metal hydrides. In general, Ni-MH cells utilize the aforementioned negative MH electrode for reversible electrochemical storage of hydrogen, and a positive electrode of nickel hydroxide (Ni(OH)2) material. Nickel hydroxide has been used for years as an active material for the positive electrode of alkaline batteries, including the nickel cadmium (Ni—Cd) batteries that have been largely supplanted by Ni-MH technology.
The negative and positive electrodes are spaced apart in the alkaline electrolyte. Upon application of an electrical potential across a Ni-MH cell, the MH material of the negative electrode is charged by the electrochemical absorption of hydrogen and the electrochemical generation of hydroxy ions OH−:M+H2O+e−←DishargeCharge→MH+OH−  (1)The negative electrode (cathode) reactions are reversible. Upon discharge, the stored hydrogen is released to form a water molecule and evolve an electron.
The reactions that take place at the nickel hydroxide positive electrode (anode) of a Ni-MH cell are:Ni(OH)2+OH−←DischargeCharge→NiOOH+H2O+e−  (2)Hence, the charging process for a nickel hydroxide positive electrode in an alkaline storage battery is governed by the following equation:Ni(OH)2+OH−→NiOOH+H2O+e−  (3)
The charging efficiency of the positive electrode and the utilization of the positive electrode material is effected by the energy-parasitic oxygen evolution process which is controlled by the reaction:2OH−→H2O+½O2+2e−  (4)During the charging process, a portion of the current applied to the battery for the purpose of charging, is instead consumed by the oxygen evolution reaction (4). The oxygen evolution of equation (4) is not desirable and contributes to lower utilization rates of the positive active material upon charging. (An analogy can be made to discrete dry MH that absorbs more gaseous hydrogen as the pressure of the hydrogen is increased, where in Ni-MH cells the “pressure” is the electrical current, or rate of hydrogen production, during charging that is proportional to the degree of utilization of the MH storage).
One reason both reactions (oxygen and hydrogen evolution) occur simultaneously is that their electrochemical potential values are very close. Anything that can be done to widen the gap between them, lowering the nickel (or anode) electrochemical potential in reaction (3) or raising the electrochemical potential of the oxygen evolution reaction (4), will contribute to higher utilization rates (higher hydrogen production rate and therefore higher absorption by the MH electrode). U.S. Pat. No. 6,017,655 is one attempt to widen this gap by use of disclosed additives to the nickel hydroxide anode. It is noted that the electrochemical potential of the oxygen evolution reaction (4) is also referred to as the oxygen evolution potential.
Another way that is disclosed herein is to separate the functions of the anode, reserving use of the nickel hydroxide (or other) anode for discharge and for supplemental charging only, while adding a photocatalytic anode, or photoanode, for charging that, when activated by light, reduces or eliminates the required electrochemical potential. Photoanodes are semiconductors, modified semiconductors, or semiconductor compounds in monolithic form, or paste on monolithic form, that absorb and then convert photons of light into pairs of surface charges that can either electrolyze water directly or reduce the voltage normally required for electrolysis. Ideally such semiconductor photoanodes have conduction and valance band edges that overlap either the hydrogen evolution potential, the oxygen evolution potential, or both. If both, then spontaneous water electrolysis occurs when the photoanode is illuminated; this is known as photolysis. Otherwise a bias voltage or overpotential voltage that is considerably less than even the theoretical 1.48 VDC electrolysis potential, is required to initiate electrolysis by bridging the gap between either band edge and its respective evolution potential, in which case the process is called photoelectrochemical hydrogen production. (For the purpose of describing the invention herein, whether the photoanode requires no bias or some non-zero bias voltage the resulting secondary cell is called “photoelectrochemical”.) For example, U.S. Pat. No. 7,485,799 to Guerra, and U.S. Pat. No. 8,673,399 to Guerra et al disclose photoanodes in which nanostructures strain a semiconductor such as titanium dioxide (titania or TiO2) such that the bandgap of the semiconductor is altered to favorably absorb more of the solar spectrum and also the band edges are more favorably aligned with the hydrogen and/or oxygen evolution potentials.
So semiconductor photoanodes, when illuminated with light having energy hv where h is Plank's Constant and v is the frequency of that light, greatly reduce or even eliminate the applied electrical potential (or voltage) required because a band edge is either close to or overlaps the oxygen and/or hydrogen evolution potential(s). Absorption of the light energy forms charge pairs of electrons (e−) and holes (p+) at the semiconductor surface (SC) as in (5) below.hv+SC→(e−+p+)  (5)Except that the electrons and positive charges arise from interaction of light with the semiconductor photoanode rather than from an external applied electrical potential, the governing reaction for oxygen evolution (6) can be seen to be identical with (4) above for a conventional anode:2p++2OH−→H2O+½O2  (6)
Because a Ni-MH battery is, in the charge mode, basically an alkaline electrolyzer, it will be seen that it can be improved by addition of a photoanode. Presently, external electricity applied to the nickel hydroxide anode and MH cathode produces, from the aqueous electrolyte, hydrogen at the cathode and oxygen at the anode. The hydrogen is immediately absorbed by the metal hydride for use to produce electricity during later discharge. (This is one of the sources of efficiency in the MH architecture in that less energy is required for the hydrogen to be absorbed by the MH than to be evolved from a “normal” cathode.) The high power density of Ni-MH batteries comes from the fact that hydrogen can be stored in metal hydrides at energy densities even greater than that of liquified hydrogen. The theoretical voltage required to split the water in the electrolyte is 1.23 VDC. However, the overvoltage that is required to overcome the oxygen potential brings the actual theoretical voltage to 1.48 VDC. An electrolyzer that can produce hydrogen with 1.48 VDC would then have an electrolyzer, or Faraday, efficiency of 100%. In practise, most electrolyzers are only 50% efficient, requiring nearly 3 VDC, to 75% efficient, requiring 2 VDC. Similarly, the electrolysis efficiency during the charging cycle in a typical Ni-MH battery is about 66%. Addition of a photocatalytic anode, or photoanode, reduces or even eliminates the amount of electrical power required to electrolyze water to produce hydrogen. Light works with the photocatalyst to either electrolyze the water directly, with no external electrical power required, in so-called “zero bias” or photolytic mode, or with significantly reduced electrical power in photoelectrochemical mode. For example, the photoanode taught in U.S. Pat. No. 8,673,399 is able to produce hydrogen from water even without external electricity, and has maximum hydrogen production at only 0.9 VDC, or half to even a third of the voltage that a conventional electrolyzer requires to produce an equivalent amount of hydrogen.
Therefore the present invention discloses a photoelectrochemical secondary cell that can employ these photoanodes as well as any other photoanodes for the charging cycle in a metal hydride battery.
Furthermore, charging of the photoelectrochemical secondary cell, or a battery comprised of multiple such cells, requires only sunlight for trickle charging. This is valuable in applications where an external electrical source that may not be readily available, such as to power telecom devices that are often in remote locations or atop towers and poles, but also for personal electronic devices or batteries for space. And while faster charging will still require an external electrical supply, that supply power requirement is greatly reduced by the properties of the photoanode.